Electroimpedance tomography (EIT) is a method in which a weak electric alternating current is fed into the human body in order to measure the surface potentials at different points of the body. By rotating the points at which the current is fed into the body while measuring the surface potentials at the same time, it is possible to determine a two-dimensional tomogram of the electric impedance distribution in the body being examined by means of suitable mathematical reconstruction algorithms. In medicine, a tomogram of the impedance distribution of the human body is of interest because the electric impedance changes both with the air content and the extracellular fluid content in the tissue. The ventilation of the lung as well as the shifts in blood and serum parameters can thus be visualized and monitored in a regionally resolved manner.
To carry out gentle respiration of the patient, it is necessary to know the functional residual capacity (FRC) of the subject's lung. The functional residual capacity is defined as the volume of gas that remains in the lung at the end of expiration. If, for example, a plurality of alveoli are collapsed, the FRC is smaller than when the alveoli are open. A frequently used method for measuring the FRC according to the so-called wash-in or wash-out method is described, for example, in EP 791 327 A1. This measurement can be carried out during mechanical respiration.
The combination of a respiration system with a measuring system for electric impedance tomography is described in DE 103 01 202 B3. The measurements at the test subject can be carried out at exactly defined points in time and as a function of the course of respiration due to the bidirectional data exchange between the respiration system and the measuring system for electric impedance tomography.
If, for example, a respiration maneuver is initiated in a patient in the form of a brief pressure rise (recruitment maneuver) as a possible method for improving ventilation, a plurality of variables should change in the successful case. Thus, an attempt is made by means of the recruitment maneuver to reinflate collapsed regions of the lungs, so that these regions will again participate in the ventilation and gas exchange will thus improve. Both the ventilation and the FRC will improve in case of a successful recruitment maneuver without already ventilated regions being compromised by the brief pressure rise. However, information on the FRC and the change in the FRC is usually unavailable. It is of little benefit if a majority of the increase in the FRC and/or of the improvement of ventilation increases in regions that were already well ventilated before the recruitment maneuver and the actually collapsed target regions hardly benefit or benefit only little from the inflating. Consequently, it is of fundamental interest for the user to know which region of the lung has contributed to the change in ventilation and FRC. The breakdown of the FRC or the change in FRC and ventilation or change in ventilation according to local contributions of various regions of the lungs is called the regional or even local FRC or FRC change and regional or local ventilation or ventilation change in this connection. This is to be understood in the sense of characteristic values within ROIs (Region of Interest). It can be assumed that it is possible to determine the regional ventilation or ventilation change by means of electroimpedance tomography. A local FRC determination or FRC change can be estimated so far only by computed tomography and partly also by X-ray, with the known disadvantageous effects of these methods on the patient. Besides the recruitment maneuver, an FRC change can also be obtained from the administration of a drug or from the recovery of the lung.